Six down, four to go in my personal selection of the ‘top ten’ classic Doctor Who stories, and now I’m getting to the stage where rather than deciding which story I might include next I’m thinking instead about which stories I can’t leave out.

So far in my top ten classic Doctor Who stories I’ve selected one personal favourite (‘The Robots of Death’) and one that’s widely regarded as an excellent example of a particular type of story (‘The Tomb of the Cybermen’). This week I’ve settled on a story that might not quite be among the best dramatically but which I’ve chosen for its overall significance for both the internal and the external history of Doctor Who.